Reach is a keyword ability defined in CR 702.17a: 'A creature with reach can block creatures with flying.' This is a complete exception to the flying rule (CR 702.9b), which normally states that a creature with flying can only be blocked by creatures that also have flying or reach.
Magic: The Gathering is an abstract game — there is no literal concept of 'on the ground' or 'in the air.' Reach simply grants the mechanical permission to block flyers. The flavor of reaching up is just that: flavor. Rules-wise, all that matters is whether the blocking creature has the reach keyword.
It's also important to note that reach does NOT grant flying. A creature with reach but not flying can still be blocked by any creature, and it cannot block other flyers unless it also has flying or reach itself — wait, actually reach does let it block flyers, but it cannot fly over ground blockers. In short, reach is purely a blocking permission for flyers, nothing more.
Concrete example: Your opponent attacks with a 2/2 Dragon with flying. You control a 3/3 Spiderbeast with reach but no flying. You may legally declare Spiderbeast as a blocker for the Dragon, because reach satisfies the flying blocker restriction (CR 702.9b, 702.17a). The Dragon is blocked and combat proceeds normally.
Citations: CR 702.9b states the flying restriction on blockers; CR 702.17a provides the full text of reach and its exception.
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